A number of prior studies in humans have shown that visual working memory capacity is limited to approximately 4 items, and this depends on the posterior parietal cortex. However little is known about the basis of capacity and its limitations at the single neuron level. In order to examine how the posterior parietal cortex encodes this information, we designed a task of visual working memory in a monkey model. We presented 1-5 visual stimuli at one of 14 locations arranged on a ring of 10 degree eccentricity. The stimulus display was presented on a computer screen twice, with a 1 second delay period between them. During the second presentation, the stimuli appeared at the same locations as in the first, though there was a 50% probability that one of them might change in shape. The monkey was required to determine if there was a change or not by making an eye movement to a blue or green choice target, respectively. We found that as the number of stimuli increased, performance declined. Capacity was estimated to be approximately 3 items. Capacity in the left and right hemifield were not independent of each other. We studied the neural basis of the phenomenon by recording neuronal activity in the posterior parietal cortex (areas 7a and LIP). We first mapped the neuron’s receptive field and then positioned the stimulus that would change either in the receptive field or at a diametric location to it, and either surrounded by or away of the rest of the stimuli. We were thus able to determine the ability of single neuron activity to decode the identity of the stimulus that changed. Our results offer insights on the neural basis of visual working memory capacity at the level of single posterior parietal neurons.